This application claims the priority of Japanese application 8-319047, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a control apparatus and control method for an engine powertrain of a vehicle, and more particularly to a control apparatus and control method for an engine powertrain of a vehicle suitably applied to a type such that an output from a lean burn engine operated selectively at a stoichiometric air-fuel ratio or at a lean air-fuel ratio is transmitted through a CVT (Continuously Variable Transmission) to wheels of the vehicle.
In a vehicle having a CVT type automatic transmission, a transmission ratio is controlled generally by using a value searched from a transmission ratio data table preliminarily stored in a memory, according to a vehicle speed and an opening of a throttle valve for controlling the amount of intake air to be introduced into an engine. The relation between accelerator pedal operation and acceleration torque in this control follows that in the control of a multistep automatic transmission. That is, when the accelerator pedal is depressed to accelerate the vehicle, the transmission ratio is increased so as to increase a drive torque, whereas when the accelerator pedal is returned to an original position, the transmission ratio is decreased so as to reduce noise and improve fuel economy because no drive torque is required, thus providing good drivability such that no discomfort is felt by the driver.
In such control, however, it is necessary to preliminarily determine control constants for various assumed operating conditions, so that much time and labor are required for working so called tuning or matching in designing. In particular, the number of steps in forward speed in a multistep automatic transmission is about 4 to 6, while the number of steps in forward speed in a CVT type automatic transmission is infinite, or continuous. Accordingly, vast amounts of time and labor are taken to create the transmission ratio data table mentioned above. To solve this problem, it may be considered to control a transmission ratio as computing an optimum transmission ratio during running, thereby greatly saving the time and labor for tuning. An example of this control is described in Japanese Patent Laid-open No. 7-174219 proposed by the present inventors.
However, an engine and engine management system such that an engine torque is varied in no proportion to the depression amount of an accelerator pedal have recently been put to practical use. Examples of such an engine include a lean burn engine, a direct injection gasoline engine, and an engine combined with an electronically controlled throttle valve. However, the above-mentioned conventional transmission ratio control cannot be applied to transmission ratio control for the combination of such an engine or engine management system and a CVT type automatic transmission.
For example, it is now assumed that the above-mentioned conventional transmission ratio control is applied to a vehicle having a lean burn engine capable of running at a lean air-fuel ratio (in a lean burn condition) and a CVT type automatic transmission. In this case, when an accelerator pedal is depressed to accelerate the vehicle in the lean burn condition of the engine, the transmission ratio is increased and the lean air-fuel ratio is changed to a stoichiometric air-fuel ratio, so as to increase an acceleration torque. At this time, the throttle valve opening is increased by the depression of the accelerator pedal to cause an increase in intake air amount. Such an increase in throttle valve opening and a change from the lean air-fuel ratio to the stoichiometric air-fuel ratio cause a rapid increase in engine torque, making the driver feel a discomfort. Further, a rapid increase in drive torque in running on a slippery road invites spinning of tires, which is dangerous.
When the acceleration is finished, the air-fuel ratio is returned to the stoichiometric air-fuel ratio to the lean air-fuel ratio, and the engine torque is accordingly decreased. Such a decrease in the engine torque causes shock, which is transmitted to a vehicle body, resulting in an uncomfortable feeling by the driver.
It is accordingly an object of the present invention to provide a control apparatus and control method for an engine powertrain of a vehicle including a lean burn engine and a CVT type automatic transmission which can obtain a comfortable acceleration feeling proportional to the depression amount of an accelerator pedal and can eliminate the shock due to changing of an air-fuel ratio.
According to the present invention, the CVT type automatic transmission is controlled by computing an engine torque according to an engine combustion condition, computing an output torque of the transmission from the engine torque, a transmission ratio and a torque ratio of a torque converter, estimating an output torque demanded by a driver from a vehicle speed and an accelerator pedal depression amount, and computing a control value of the transmission ratio so that a deviation between the output torque computed and the output torque demanded becomes small. As a result, a comfortable acceleration feeling can be obtained and the shock due to changing of an air-fuel ratio can be suppressed.